In general, the otter board used in trawl fishing having as large as possible value of L/D, i.e. having a large net-mouth expanding force (L) and/or a small drag (D) is desirable. When these requisites are satisfied, the drag of otter board is decreased, which enables tugging of large trawl net and using of the otter board having smaller surface area with easier handling and lower costs.
In developing the heretofore known otter boards, a number of attempts have been carried out for making the value L/D as large as possible.
For example, a slit type of otter board is suggested which has a plurality of longitudinal slits 2, 2 . . . in the concave inner surface thereof as shown in FIG. 5, or a multi-wing type of otter board is proposed which comprises several wings 4, 4 . . . having each a large ratio of longitudinal and lateral sizes.
The first type of otter board having slits 2 therein has an substantially decreased surface area thereof, so that the drag D thereof is decreased but its net-mouth expanding force (L) is also decreased, while the second multi-wing type of otter board has an increased net-mouth expanding force (L) but its drag (D) is also increased.